1. Field of the Invention
The present invention relates to a numerical controlling unit having a highly accurate tool-breakage detecting function.
2. Description of Related Art
A tool such as a drill is generally attached to a spindle. The tool is rotated by rotating the spindle so that a workpiece is cut by means of a relative speed between the spindle and the workpiece. The spindle is driven in rotation by a rotation of a spindle motor. Thus, when cutting a workpiece, a load caused by the cutting is indicated as an electric current of the servomotor.
If a tool, in particular a cutting edge of a tool, breaks or fails during cutting, a load caused by the cutting is changed, and thus a current of the servomotor is changed. By previously setting a current value indicating a load at a breakage of the tool (cutting edge of the tool) as a reference value, and monitoring the current value of the servomotor which drives a spindle, a tool breakage can be detected when the monitored current value becomes higher than the reference value.
A tool may have a larger or smaller load to be detected depending on cutting conditions such as feed speed and cutting amount.
Although there are different cutting conditions as described above, a higher torque value than a load torque value at an actual tool breakage is used as a reference value so as not to incorrectly detect a breakage. Therefore, when a tool breaks at a cutting step of a lower load, there is the possibility that the tool breakage cannot be detected, because the load torque value does not become higher than the reference value.
In machining a mold, since a load changes greatly, it is difficult to detect a tool breakage by a method in which a single reference value is set per tool.
Japanese Patent Laid-Open Publication No. 2000-235411 (Numerical Controlling Unit Using Machining Information) discloses an invention in which additional tool information such as CAM (Computer Aided Manufacturing) information is added to a machining program, and the information is inputted into a numerical controlling unit so as to control a speed.
A load during cutting can be known by means of the numerical controlling unit according to the above invention.